1. Field of the Invention
This invention relates to the field of dental cement compositions, and in particular to two-component glass ionomer type dental cements which are novel in form, composition, and method of handling, featuring improved strength, handling, and reproducibility of properties over conventional glass ionomer cements.
2. Background of the invention
Prior art glass ionomer cements are formulated as a two-part system, one part being in a liquid form and the other in a powder form. The liquid is typically a solution of oligomers or copolymers of acrylic acid. The molecular weight of such polymers is usually in the range of 40,000 to 50,000, and their concentration may vary from about 40% to 60%. The powder is composed of fine alkaline glass particles, and its chemical composition includes, as essential ingredients, silicon and aluminum oxides, calcium fluoride, and modifying additives which may include aluminum, sodium or barium fluorides, alkaline metals or alkaline earth oxides, aluminum phosphate, zinc, zirconium, and titanium oxides.
Powder/liquid systems are the least desirable forms of self (chemically) cured dental cements and restoratives. Maintaining the proper proportion between the two components of a two-part cement system is, in the case of glass ionomers, critical for achieving acceptable reproducibility of the relevant characteristics of the cured material. However, it is extremely difficult to meet such a requirement with powder/liquid systems, taking into consideration the small amounts of materials involved in the preparation of mixes for dental applications and the imprecise tools used for these purposes.
Moreover, dental assistants and clinicians are accustomed to other powder/liquid type cements and restoratives which do not require a high level of precision in dispensing, and have little understanding of the difference in requirements when glass ionomer type materials are involved. Imprecise dispensing may, however, have a detrimental effect on the mechanical properties, resistance to the oral environment, curing characteristics, ability to bond to dentin and tooth enamel, and tissue compatibility of glass ionomer cements.
Generally, an excess of liquid will result in slower setting cements which are more susceptible to deterioration when exposed to saliva and more likely to irritate pulp and mucosa. Also, due to their light consistency, such mixes are unsuitable for applications requiring firmer consistencies and greater mechanical strength.
An excess of powder causes mixes to be too dry and may not allow sufficient working time. The consistency of such mixes may make them unsuitable in applications where flowability of the mix is mandatory, such as in a capacity as cavity liners, orthodontic band cements, and crown and bridge cements. Also, after cure, such cements are likely to be excessively brittle and their ability to bond to the tooth structure will be impaired.
Minor variations in the characteristics of the liquid or powder, such as variations in the molecular weight of the polyacrylic acid and particle size of the glass, may make the originally designed dispensing system unsuitable. Moreover, changes in ambient temperature influence the viscosity and surface tension of the liquid and, consequently, affect the powder/liquid ratio because of variations in drop sizes when dispensed in the customary way from dropper-type bottles. The usual way of dispensing powder by scoop represents an intrinsically imprecise method because the bulk density of the powder may vary with time due to settling and the way the powder is handled (shaking, vibration, pounding, etc.). All these factors may significantly affect the properties and, in some instances, the safety of the cements to such a degree that their suitability for an intended purpose may be questionable.
Additional problems related to variations in the particle size of the powder may also be encountered. Manufactured batches of powders consist of difficult to reproduce blends of particles of different sizes. Variations in the distribution of particle sizes are virtually unavoidable within batches of commercial products. Larger particles tend to migrate to the bottom of the container, leaving smaller particles on top. Using the same dispensing system for powders consisting of different-sized particles will result in mixes of varying consistencies and unpredictable working and setting times. It is generally recognized that smaller sized glass particles will shorten the working time and result in mixes characterized by denser consistencies.
A common characteristic of glass ionomer compositions is their undesirably short working time. In order to assure the best properties of the cured cement, mixing of the components and application should be accomplished before the blend starts to show signs of setting. However, preparation of powder/liquid mixes is time-consuming, leaving clinicians with little latitude to complete the application within the allowed working time for the cement.
Powder/liquid systems are also undesirable from the point of view of economy because substantial waste of the material is unavoidable. Dispensing cannot be accomplished in a way which closely approximates the amount of material the clinician needs. Usually, a large part of the cement is wasted.
To alleviate the shortcomings of powder/liquid versions of glass ionomers, a solution has been offered, derived from a technique used in packaging more expensive brands of dental amalgams. This system is comprised of a two-compartment capsule, separated by a breakable diaphragm. One of the compartments is filled with a measured amount of the powder, and the other with the liquid component of the glass ionomer cement. After the diaphragm is broken, the capsule is vigorously shaken using a vibrator-type machine for a specified period of time, producing relatively homogeneous mixes of more consistent quality. This solution eliminates some of the shortcomings of conventionally dispensed two-part glass ionomer cements, assuring better reproducibility of the properties of the cured cements and simplifying handling. However, it dramatically increases the cost per application and further increases waste. Also, handling of the material, although much easier when compared to individually dispensing the powder and liquid components, still remains complex, and the working time remaining after removal of the capsules from the vibrator is unconveniently short.
Attempts to formulate glass ionomer cements in a different form than the conventional powder/liquid system were, up to now, unsuccessful. The major advantages of glass ionomers in clinical applications are their ability to bond to the tooth structure, without the necessity of acid etching, and to protect the tooth structure from decay because they provide a sustained release of fluoride. Preservation of these characteristics, combined with the need to meet requirements related to mechanical strength, curing characteristics and safety, has imposed severe restrictions on the chemical composition of the cement components, their concentration, molecular weight, and physical form. These factors also severely limited freedom in incorporating various additives which, although highly desirable otherwise, were believed to have a detrimental effect on the properties of the cement, because of the necessity of operating in a narrow range of parameters and sensitivity of the fundamental components to even minor changes in their chemical composition, physical form, and concentration in the final mix.
Efforts have been undertaken to change the physical form of commercial glass ionomer cements in order to make them more convenient to use, which resulted in modifications of their compositions. These modified cements, while encompassing some of glass ionomer's fundamental components, have differed, however, from the original concept of glass ionomers in important aspects, including their basic chemistry and curing mechanism. Consequently, the major advantages of glass ionomers, including their ability to bond to the tooth structure, to maintain a high level of Fluoride release, and to prevent tooth decay, were severely compromised.
Most common examples of such modified formulations are comprised of blends of methacrylate monomers with glass ionomer-type powders used as fillers. They represent light-cured one-component or self- (chemically-) cured two component systems. Their mechanism of cure relies, however, on chain-forming of ethylenically unsaturated methacrylate monomers, while the curing of glass ionomers depends on the reaction of the carboxylic group present in polyacrylic acid with alkaline sites of glass powder. This distinctive mechanism of cure and the presence of water in glass ionomer formulations are critical to preserving their main advantages: their ability to bond to an unconditioned tooth structure and to provide sustained Fluoride release.
Some prior publications relating to the field of this invention include the following U.S. Pat. No. 5,520,922 issued May 28, 1996 to Gasser, Oswald and Guggenberger, Rainer; U.S. Pat. No. 5,520,725 issued May 28, 1996 to Kato Shin-Ichi et al.; U.S. Pat. No. 5,382,284 issued Jan. 17, 1995 to Thomas J. Arnold; U.S. Pat. No. 5,367,002 issued Nov. 22, 1994 to Huang Chin-Teh et al; and U.S. Pat. No. 5,063,253 issued Nov. 5, 1991 to Akahane Shoji et al.